Spectroscopic Evidence for a 4-Methylidene Imidazol-5-one in Histidine and Phenylalanine Ammonia-Lyases We thank Prof. G. E. Schulz and Dipl.-Chem. M. Baedeker (University of Freiburg) for providing us with the new PAL expression system and Dr. M. Stieger (Hoffmann-La Roche, Basel, Switzerland) for vector pREP4-GroESL carrying the HSP-60 system. D.R. and D.M. thank the Land of Baden-Württemberg for a scholarship for

Abstract

HAL and PAL are closely related enzymes: They both contain a recently discovered highly electrophilic prosthetic group (shown in the picture) at the active site. This observation adds weight to the previously published hypothesis that the reactions catalyzed by HAL and PAL (histidine and phenylalanine ammonia-lyases) have analogous mechanisms.